1. Field of the Invention
This invention relates to magnetic recording and reproducing apparatus, particularly to video recorders and reproducers of the helical scanning type in which a magnetic record/playback head is operatively supported within a rotating drum for scanning a magnetic tape along a plurality of adjacent discrete tracks oriented at an angle relative to the lengthwise direction of the tape, and in which special track following features are provided to enable reproduction of pictures at normal speeds, fast and slow forward and reverse speeds, and at stop motion.
2. Description of the Prior Art
Helical scan video tape recorders embodying automatic track following features to compensate for non-uniform tape motion, tape stretching, and the like are becoming well known. Such recorders are, for example, disclosed in U.S. Pat. Nos. 4,143,405 (Kubota/Sony) and in 4,151,570 (Ravizza/Ampex).
In addition to such patents wherein track following is disclosed, it is also becoming known to provide helical scan video recorders with systems enabling the playback head to follow a selected helical track, regardless of the tape speed, be it normal playback forward, fast fowward, slow motion, stop motion and reverse. For example, patents disclosing recorders enabling such special effects are disclosed in U.S. Pat. Nos. 4,163,993 (Ravizza) and 4,233,637 (Kubota/Sony).
In a conventional helical scan video tape recorder, the rotary transducer or head assembly is provided with one or more magnetic transducers adapted to scan successive parallel tracks on a magnetic record tape so as to record and/or reproduce video signals along such tracks. In general, one or more of the transducers is caused to rotate, such as within a drum assembly so as to scan diagonally across the tape, while the tape itself is transported longitudinally. A typical helical scan VTR thus preferably includes at least one record or playback transducer within a drum assembly about at least a portion of which is wrapped the magnetic record tape. Typically, during recording, a servo system will be provided to ensure uniform motion of the tape and to control the rotation of the transducer with respect to the tape movement, while control pulse signals are recorded on a portion of the tape. During normal reproduction, a similar servo control system will be used to synchronize the movement of the tape and control the rotation of the head assembly in accordance with the recorded control pulse signals.
In recently disclosed VTR's capable of various kinds of reproducing motions, such as stop or still motion, slow motion, or quick and fast motion, the tape speed obviously differs from the tape speed utilized during recording. Consequently, during playback, the scanning path of each of the playback heads within the drum assembly is different from the path followed during recording. That is, the scanning path is inclined or differently angularly disposed with respect to the recorded track. As the result of such angular differences, in the various reproducing modes other than normal speed, guard band noise, cross talk, picture jumping, and the like will result from the tracking errors due to inclination of the scanning paths of the head relative to the recorded track.
Accordingly, for example, such as disclosed in U.S. Pat. No. 4,080,636 (Ravizza), such tracking errors are adapted to be corrected by mounting the playback head on an adjustable support assembly such as a piezoelectric reed which is adapted to deflect or flex in a direction generally transverse to the diagonal track along the tape. By means of a suitable error correction signal applied to the piezoelectric element, the head is appropriately deflected transverse to the tape track to cause the scanning path to coincide with the parallel record tracks.
Helical scan VTR's adapted to create special altered time base reference effects have not been particularly successful to date because of the spurious noise generated during playback due to the transducing head crossing from one track to another. For example, slow motion effects and video recording necessarily require that the data on one track, typically a full video field per track, be repeated one or more times during the playback so that the visual motion is slowed down. Similarly, for fast forward motion, one or more tracks must be skipped. The resultant path that the transducing head follows along the tape during such reproduction processes will therefore be substantially different than the recorded track that was made during the recording process. Thus, for example, an apparatus is disclosed in Belgian Pat. No. 852,626 (Hathaway et al), which permits the tracks to be accurately followed even though the transport speed varies within wide limits. While the apparatus there described represents a significant improvement over other helical scan VTR's in that it proposes to afford the capability of reproducing special motion effects, it similarly creates transient conditions resulting in noise bars in the video image under certain operating conditions.
In order to achieve such special motion effects by appropriately deflecting the playback head transverse to the diagonal tracks, it has been previously disclosed to generate a ramp type of signal which is coupled to a head deflection assembly in order to gradually increase the deflection of the head transversely in an amount proportional to the difference between the recorded tape path and that followed by the head. Thus, for example, in U.S. Pat. No. 4,233,637, an apparatus is depicted in which pulse generators are positioned adjacent a head drum assembly to provide pulses indicative of the head position, which pulses are then coupled to a ramp generator, together with appropriate additional pulses from stepdown counters and the like in order to provide the requisite ramp signal. In contrast, in Belgium Pat. No. 852,626 referenced above, such ramp signals are provided as the result of an integrator circuit, the input to which is driven in accordance with pulse generators responsive to a trigger pulse indicative of the end of each video scan and forward and reverse tape direction detectors.